The present invention relates to a method for distilling a polymerizable compound or a liquid containing a polymerizable compound, and more particularly relates to a method for distilling a polymerizable compound such as (meth)acrylic acid or a liquid containing such a polymerizable compound with use of a distillation column equipped with a reboiler through a distillation process that is kept stable for a long period of time by effectively preventing formation of a polymer in reboiler tubes, and hence, fouling of the tubes.
Conventionally, distillation and purification of a polymerizable compound such as (meth)acrylic acid or a liquid containing such a polymerizable compound with use of a distillation device equipped with a multi-tube reboiler that is composed of a plurality of reboiler tubes fixed to tube sheets disposed inside the reboiler shell is widely practiced in industry.
Conventionally, a distillation device equipped externally with a multi-tube reboiler of a forced circulation type is widely used, as the foregoing distillation column equipped with the multi-tube reboiler. This distillation column is designed so that a pump disposed at a bottom of the distillation column is actuated with electric power or the like to pump up a liquid to be distilled (liquid to be processed, hereinafter referred to as process liquid) from the column bottom of the distillation column via the pump so as to forcibly circulate through the multi-tube reboiler.
To obtain a necessary ability, such a forced-circulation-type multi-tube reboiler sometimes requires circulation of a greater quantity of the liquid, a greater pass number, and the like.
The liquid supplied from the distillation column to the reboiler has been heated to a boiling point, and it partly evaporates due to pressure drop occurring when passing through the reboiler tubes. Therefore, distillation using the forced-circulation-type multi-tube reboiler undergoes a high ratio of evaporation of a liquid, thereby causing a polymerizable compound to easily polymerize. Accordingly, in distillation by using such a forced-circulation-type multi-tube reboiler, a polymer of a polymerizable compound tends to be formed thereby fouling surfaces of tubes, and hence, causing a problem of plugging.
A well-known scheme to cope with the foregoing problem is a scheme in which a valve or the like is provided at an outlet nozzle of a multi-tube reboiler (a tube connecting a reboiler shell of the multi-tube reboiler and a distillation column) so as to make a temperature inside the reboiler tubes of the multi-tube reboiler not higher than a boiling point. This scheme, however, does not completely solve the foregoing problem, due to the following drawbacks: channeling; a lower fluid velocity in a chamber; and a longer retention time. In case a polymer is formed and fouls tubes, the operation is suspended and the polymer is manually or chemically removed.
Furthermore, recently, from the viewpoint of energy saving, distillation is practiced by using a distillation column equipped with a multi-tube reboiler 52 of a natural circulation (thermosiphon) type shown in FIG. 6 that does not need electricity as power.
In the foregoing distillation in which a distillation column equipped with a natural-circulation-type multi-tube reboiler 52 is used, a liquid supplied to the multi-tube reboiler 52 from the distillation column 51 is heated in the multi-tube reboiler 52 thereby partially vaporizes. This causes density inside the distillation column 51 to become different from that inside the multi-tube reboiler 52, and hence, causes the process liquid to naturally circulate from the column bottom of the distillation column 51 to the multi-tube reboiler 52 with a lower density.
The distillation in which the natural-circulation-type multi-tube reboiler 52 is used is discussed by, for example, xe2x80x9cTroubleshooting distillation columnsxe2x80x9d Chemical Engineering, Jul. 31, 1978, pp. 70-78. In this, a liquid level 57 in the distillation column 51 is considered from the viewpoint of efficiency of heat transfer in the distillation system.
As described in the foregoing reference, in the case where distillation is carried out with use of the distillation column 51 equipped with the natural-circulation-type multi-tube reboiler 52, the liquid level 57 in the distillation column 51 is maintained near the top tubesheet 53 of the multi-tube reboiler 52.
This is because, in the case where distillation is carried out with use of the distillation column 51 equipped with the natural-circulation-type multi-tube reboiler 52, the following problems are deemed to occur if the liquid level 57 in the distillation column 51 is set higher or lower than the top tubesheet 53 of the multi-tube reboiler 52 to which reboiler tubes 56 are fixed.
Namely, when the liquid level 57 in the distillation column 51 is lower than the top tubesheet 53, vaporization tends to increase, making the natural circulation difficult or impossible.
On the other hand, when the liquid level 57 in the distillation column 51 is higher than the top tubesheet 53, a heat transfer coefficient lowers, and further, in some cases, the reboiler outlet nozzle 54 becomes flooded with the liquid, and hydraulic hammer occurs.
Therefore, considering the heat transfer coefficient, in the case where distillation is carried out with use of the distillation column 51 equipped with the natural-circulation-type multi-tube reboiler 52, to avoid setting the liquid level 57 in the distillation column 51 higher or lower than the top tubesheet 53 of the multi-tube reboiler 52 is deemed preferable.
However, in the case where a polymerizable compound or a liquid containing a polymerizable compound is distilled with use of the above-described distillation column 51 equipped with the natural-circulation-type multi-tube reboiler 52, formation of a polymer in the reboiler tubes 56 cannot be avoided even in the case where distillation is carried out in a state in which the liquid level 57 in the distillation column 51 is maintained substantially as high as the top tubesheet 53 in the multi-tube reboiler 52. Therefore, as described above, when the tubes are fouled, the operation has to be stopped so that the polymer is removed manually or chemically.
An object of the present invention is to provide a method for distilling a polymerizable compound such as (meth)acrylic acid or a liquid containing such a polymerizable compound by using a distillation column equipped with a reboiler, by which method the distillation can be stably carried out for a long time of period by preventing formation of a polymer in reboiler tubes.
The inventors of the present invention made eager studies to achieve the above-described object, and discovered that the foregoing object could be achieved by maintaining a liquid level in the distillation column at a specific height by using a specific vertical multi-tube reboiler. Based on this finding, the inventors successfully completed the present invention.
More specifically, to achieve the foregoing object, a method for distilling a polymerizable compound or a liquid containing a polymerizable compound of the present invention is a method for distilling a polymerizable compound or a liquid containing a polymerizable compound by using a distillation column equipped with a reboiler, in which (a) as the reboiler, a vertical multi-tube reboiler whose outlet nozzle inside diameter (D1) is 0.2 to 1 time its reboiler shell inside diameter (D2) is used, (b) an outlet of an outlet nozzle of the reboiler to a gas-phase section of the distillation column is provided at a position satisfying:
L=(0.4 to 3)xc3x97D1
where L is a distance from a top tubesheet of the reboiler to a lower end of an outlet of the outlet nozzle, and D1 is the outlet nozzle inside diameter, and (c) a liquid level in the distillation column is maintained in a range satisfying:
H=(0.1 to 0.8)xc3x97L
where H is a height of the liquid level from the top tubesheet of the reboiler, L is a distance from the top tubesheet of the reboiler to the lower end of the outlet of the outlet nozzle, and D1 is the outlet nozzle inside diameter. By the foregoing method, a boiling point of the polymerizable compound or the polymerizable compound containing liquid in the reboiler is allowed to become higher than in conventional cases.
In other words, to achieve the foregoing object, in distillation of a polymerizable compound or a liquid containing a polymerizable compound by using a distillation column equipped with a vertical multi-tube reboiler, for suppression of polymerization of a polymerizable compound, an outlet of an outlet nozzle of the reboiler to a gas-phase section in the distillation column is provided at a position in accordance with an inside diameter (D1) of the outlet nozzle of the reboiler, and a height of a liquid level in the distillation column is maintained lower than a lower end of an outlet of the outlet nozzle and higher than a top tubesheet of the reboiler, so that a boiling point of the polymerizable compound or a liquid containing the polymerizable compound in the reboiler is raised.
With the foregoing arrangement, formation of a polymer in the reboiler tubes is effectively prevented, and distillation of a polymerizable compound or a liquid containing a polymerizable compound can be stably carried out for a long time of period.